1. Field of the Invention
The present invention relates to methods and apparatus for superplastic forming of metals, and more particularly to novel means for effecting superplastic forming of metals at temperatures greater than 1000.degree. C., while resisting oxidation and creep deformation ordinarily attendant conventional tooling.
2. Background of the Invention
For many years, it has been known that certain metals, such as titanium alloys, exhibit superplasticity within limited temperature ranges and strain rates. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency towards necking. Thus, when in a superplastic condition, the metal alloy exhibits low resistance to deformation and may be elongated with reduced thinning. This permits a sheet of such metal to be readily formed against dies to achieve desired shapes while maintaining a substantially uniform thickness in the finished part without any weak points.
Superplastic forming (SPF) may be performed in conjunction with diffusion bonding, a process in which the surfaces of facing metallic components are metallurgically joined through application of high temperatures and pressures sufficient to cause commingling of atoms at the junction of the surfaces.
Conventional superplastic forming (SPF) techniques are limited to temperatures below approximately 1000.degree. C. This temperature restriction is due to excessive oxidation of parts and tooling components, as well as creep deformation of metallic dies when subjected to press loads during forming. In addition, a significant need has developed for SPF processes which can use temperatures up to approximately 1250.degree. C. for fabrication of materials such as gamma titanium aluminide materials.